Oxide dispersion strengthened (ODS) ferritic stainless steel alloys are considered excellent material candidates for future generation power systems, due to optimum thermal, mechanical, and nuclear properties [references 1-4]. Gas atomization reaction synthesis (GARS) has previously been demonstrated as a feasible rapid solidification method for the production of precursor ODS ferritic stainless steel powder [reference 5]. During this process, nascent atomized droplets react with small amounts of O2 within the reactive atomization gas to form an ultra-thin (t<50 nm) surface oxide film (e.g., Cr2O3), [reference 6].
The rapidly solidified GARS powders contain a distribution of Y-enriched intermetallic compound (IMC) precipitates. Heat treatment of the consolidated powders results in an oxygen exchange reaction between the Cr-enriched prior particle boundary (PPB) oxide and Y-enriched IMC precipitates. For this reason, the IMC solidification pattern was found to be a template for the resulting nano-metric Y-enriched oxide dispersoids [reference 8]. The most ideal spatial distribution of Y-enriched IMC precipitates was found in ultra-fine powders (dia.<10 μm), which provided motivation to improve the yield of such powders. Furthermore, an ideal balance between Y and O, based on the stoichiometry of the resulting oxide dispersoids, is required to fully dissolve the PPB oxide [reference 5]. This ideal balance is typically only possible across a narrow range of gas atomized powders, since the O is in the form of a surface oxide and therefore varies as a function of particle surface area [reference 9], which provides further incentive to narrow the resulting powder standard deviation, in order to maximize powder yield containing an ideal Y to O ratio.
The present invention resulted from applicants' effort to increase the yield of ultra-fine powder (i.e., dia.<10 μm) and reduce the resulting powder standard deviation (i.e., d85/d50) using a high pressure gas atomization (HPGA) nozzle modified with the intent of enhancing the intensity of the closed-wake gas structure to promote a more prolonged and effective secondary break-up process by confining the molten metal within the recirculation zone and forcing the exiting liquid droplets to traverse the Mach disk. To this end, the close-coupled atomizing nozzle pursuant to the invention contains two concentric rings of discrete gas jets that are supplied from independent gas manifolds, which features are not present in the original design of the discrete jet HPGA nozzle (DJ-HPGA) introduced by Anderson et al. [reference 10].
The original DJ-HPGA nozzle operates with under-expanded gas jets that freely expand as they exit their individual cylindrical passages by means of expansion and compression waves, (Prandtl-Meyer fans), as explained by Espina and Ridder [reference 11]. These expansion and compression waves are reflected at the constant pressure boundary and axis of symmetry, respectively (see FIG. 2a). Above a certain pressure threshold, the reflected waves combine together and form incident oblique shocks. These incident shocks converge, forming a shock node that produces two reflected shocks, with one shock reflected toward the boundary layer and the other toward the axis of symmetry (see FIG. 2a) [reference 11]. The latter of these reflected shocks will continue to bend and flatten prior to intersecting the axis of symmetry, resulting in the formation of a Mach disk. The formation of the Mach disk truncates the recirculation zone and isolates the wake region, resulting in deep aspiration at the exit orifice of the melt delivery tube [reference 12]. This phenomenon generally occurs at a specific pressure for a given nozzle geometry, gas type (e.g., Ar or N2), and melt delivery tube geometry (e.g., extension length and angle) [reference 13].
The Mach disk is thought to play a germane role in the production of fine powder, both directly and indirectly, as it creates a barrier supported by highly focused gas that isolates the wake region from a high pressure stagnation front [reference 14]. Liquid fragments or droplets are abruptly decelerated as they pass through the Mach disk and crash into the high pressure stagnation front, which helps to further disintegrate the liquid into a fine mist. Consequently, when the Mach disk is disrupted, high pressure from the stagnation front rushes into the low pressure recirculation zone and impedes the liquid stream descent, which forces the liquid to bloom and spread or film across the transverse landing of the melt delivery tube prior to being sheared by supersonic atomization gas along the periphery of the tube (see FIG. 2b) [references 15, 16] This pre-filming action has been suggested as a plausible reason for improved fine powder production under closed-wake conditions [reference 15]. This notion agrees well with the fundamental concept of the acceleration wave model [reference 17], inferring that prefilming helps to maximize the mismatch velocity between the liquid and atomization gas and also reduces the melt layer thickness, both of which promote a reduction in resulting average droplet diameter. Moreover, this temporary disruption in liquid flow allows the Mach disk to reestablish, creating deep aspiration that again pulls liquid (e.g., fragments and droplets) into the Mach disk, thus restarting the cycle and giving rise to the term pulsatile atomization. Mullis et al. [reference 18] have empirically studied this pulsation effect using an image analysis routine to evaluate high-speed video stills, in order to calculate the frequency of the pulses and relate them to atomization efficiency in terms of the atomizer being in an open or closed-wake condition. However, a challenge still exists in understanding and controlling this pulsation effect (e.g., changes in frequency and effectiveness of the melt interruption), and how it relates to the resulting particle size distribution.